requests下载图片太麻烦了，虽然urlretrieve可以一步搞定，但是我想统一使用requests库。
注释部分就是用requests写的。
from urllib.request import urlretrieve
import requests
from bs4 import BeautifulSoup
r = requests.get("http://www.pythonscraping.com")
bs = BeautifulSoup(r.text)
image = bs.find("a", {"id": "logo"}).find("img")["src"]
urlretrieve(image, "logo.jpg")
# ir = requests.get(image, stream=True)
# if ir.status_code == 200:
# with open('logo.jpg', 'wb') as f:
# for chunk in ir:
# f.write(chunk)
